Serial Digital Interface (SDI) adaptive cable equalizers are known in the art. One example of a known SDI equalizer 10 is illustrated in FIG. 1. The example SDI equalizer 10 includes a plurality of equalizing gain stages 12, an automatic gain control (AGC) feedback loop 14, a quantized feedback (QFB) DC restoration circuit 16, an output buffer 18 and a carrier detect circuit 20.
The plurality of gain stages 12 are gain controlled by the AGC feedback loop 14 to apply an optimum amount of gain to the SDI input signal. In order to adjust the gain to an optimum level, the AGC feedback loop 14 typically compares the output of the gain stages 12 (AGC Input) with a quantized reference signal (AGC Reference) to generate an error signal that is used to adjust the gain of the plurality of gain stages 12.
The QFB DC restoration circuit 16 is used to recover low-frequency components of the SDI input signal. In a serial digital data communication system, low-frequency signal components are often lost when the signal passes through a high-pass filter, such as an AC-coupling network. Due to the wide frequency content of the SDI signal, the AC-coupling removes the low-frequency contents of the signal that could convey useful information. Quantized feedback (QFB) is a well-known and proven technique for implementing a DC-restoration function to recover the low-frequency components of the SDI signal.
As illustrated in FIG. 2, the QFB DC restoration circuit 16 operates by AC coupling (22) the input signal into a slicer 24 that employs a DC feedback loop 26 around itself. A detailed description of a QFB DC restoration circuit is provided in commonly-owned U.S. Pat. No. 6,463,108, titled “Latch-Up Recovery In Quantized Feedback DC Restorer Circuits,” which is incorporated herein by reference in its entirety. In operation, the QFB DC restoration circuit 16 introduces a hysteresis when the QFB slices the signal. The hysteresis originates from the feedback around the slicer as shown in FIG. 2. This hysteresis implies a dead zone in the transfer characteristic of the QFB that prevents small signals from being able to pass the threshold levels, trigger the latch, and create fluctuations at the QFB output.
With reference again to FIG. 1, the quantized output of the QFB DC restoration circuit 16 is output through a buffering output stage 18 and is also fed back as a reference signal (AGC Reference) for the AGC loop 14. Even though an independently and internally-generated DC value could be used as the AGC reference signal, it is typically preferred to use the transition amplitudes of the QFB output as the AGC reference because this signal closely matches the launched signal variations and provides the best AGC operation for almost any kind of input signal independent of signal pattern. The drawback of using the QFB output in this manner. however, is that the operation of the AGC loop is made dependant on a reference signal that is only available if an input signal exists and is amplified enough to guarantee signal crossings beyond the QFB dead zone levels caused by the hysteresis. For example, if no signal is applied to the equalizer, then the QFB output becomes silent and therefore does not generate the AGC reference. In this situation, the AGC will wrongly converge the gain to its minimum value in an attempt to match the signal to the reference and force a lock-up situation.
To prevent the lock-up problem explained above, and as shown in the block diagram of FIG. 1, the traditional implementations have added a carrier detect circuit 20 that disables the AGC loop 14 when no signal fluctuations are observed and ramps-up the gain until such fluctuations start to appear. In order for this added functionality to operate properly, however, the detection threshold level of the carrier detect circuitry needs to be above the QFB threshold levels due to the hysteresis. This new requirement imposes more design, characterization, test, field application, and manufacturability constraints, and has shown to be problematic in many practical cases. It is therefore desirable to provide an adaptive equalization system having an improved QFB DC restoration circuit that provides lock-up-free operation without requiring the use of a carrier detect feedback.